Hot Stamping Tools Research Articles

Novel tool steel for application in hot stamping

Hot stamping is a commonly used procedure to manufacture components from high strength steels, which are used for parts of car bodies. Against the background of production efficiency and costs, there is a number of factors which determine the performance of tools used in hot stamping, e.g. cycle time in production and lifetime of the tools. Main requirement is a high wear resistance, while additional properties such as high thermal conductivity can be considered being beneficial. Many steel grades, which are currently applied in hot stamping tools (e.g. hot work tool steel 1.2367) contain high amounts of alloying elements which gives them good mechanical properties, particularly at high temperatures. However, these steels in turn suffer from a rather low thermal conductivity and only moderate wear resistance. Optimization of the balance of mechanical properties, wear resistance, and thermal conductivity can lead to a better general performance of hot working tool steels used in hot stamping applications. In this study, a novel tool steel is characterized with special focus on properties relevant for hot stamping tools. Results are compared to those of reference hot work tool steels. Wear tests that were conducted in the framework of this paper include laboratory setups like ASTM G65 / G75 and high-temperature strip-drawing test setups to simulate the conditions of the process closer to reality. Results show that the novel tool steel is characterized by an increased wear resistance in combination with a higher thermal conductivity compared with common hot work tool steels. Furthermore it is approved that a wide range of desired properties can be adjusted in this alloy via heat treatment. The favourable combination of properties makes the novel tool steel suitable for application in hot stamping tools.

Hybrid laser metal deposition of a Fe–Cr–Mo–V–Mn tool steel for hot stamping applications

In this investigation, a novel Fe–Cr–Mo–V–Mn hot work tool steel powder was specifically developed for laser-based additive manufacturing, targeting the possibility to create conformal cooling channels in hot stamping tools for improved cooling efficiency during the forming of steel sheets for the automotive industry. Specimens of the proposed tool steel were printed via laser metal deposition and characterized to demonstrate its compatibility with hot work tool steels that are commonly used in the tooling industry. The applicability of the developed material was proved by fabricating cooling channels in a simple geometry demonstrator using a hybrid process combining milling and laser metal deposition. Finally, a hybrid-manufactured hot stamping tool segment was tested in a pilot plant to evaluate the effect of the investigated material on the cooling performance when compared to a conventional tool machined from H13 hot work tool steel. The results showed that the Fe–Cr–Mo–V–Mn tool steel features thermophysical properties similar to the most popular H13 steel and it can be efficiently used to produce tools containing conformal cooling channels by hybrid laser metal deposition, without the need of intermediate structures to improve the compatibility between the substrate and the deposited volumes.

Application of plastometry technique for hot stamping tool steels

Application of plastometry technique for hot stamping tool steels

Investigation on the thermo-mechanical properties of hot stamped parts by using laser-implanted tool surfaces

In the automotive industry, hot stamping has been established as a key technology for manufacturing safety-related car body components with high strength-to-weight ratio. During the forming operation, however, hot stamping tools are highly stressed by cyclic thermo-mechanical loads, which encourage the formation of severe wear and high friction at the blank-die interface. Against this background, an innovative surface engineering technology named laser implantation has been investigated for improving the formability of the parts and the efficiency of the hot stamping process. The laser implantation process is based on the generation of highly wear resistant microfeatures on tool surfaces by embedding hard ceramic particles via pulsed laser radiation. As a consequence, the contact area of the tool and thus the tribological and thermal interactions at the blank-die interface are locally influenced. In previous studies, the improved tribological performance of the modified tool surfaces has already been proven. However, the thermal interactions between tool and workpiece have not been analyzed, which in turn have a significant impact on the resulting part properties. In this regard, quenching tests have been carried out under hot stamping conditions by using conventional as well as laser-implanted tooling systems. Based on these results, Vickers hardness test and optical measurements have been performed on the quenched blanks, to qualify the mechanical properties and clarify the cause-effect relations.

Manufacturing of Integrated Cooling Channels by Directed Energy Deposition for Hot Stamping Tools with Ball Burnished Surfaces

Hot stamping tools require cooling channels, preferably with a high positioning flexibility. Conventionally, these are machined. This represents a disadvantage because of the limited accessibility for milling tools and the low flexibility. By means of the Directed Energy Deposition (DED) process, a flexible design of the cooling channels is possible. Different geometries of cooling channels can be manufactured by DED in order to control the heat balance in the hot stamping tool. In this context an agreement between the additive producibility and the surface fraction of the cooling channels, which contributes to the effective heat at the tool surface, is important. Experimental and numerical analyses demonstrate that a possible configuration in this field is the drop shaped cooling channel. To reduce the surface roughness after the DED process, the tool surfaces are ball burnished subsequently. The resulting roughness and the waviness of the tool surface are reduced but not leveled completely. Texturing of the surface can be applied to in fluence the material flow in the hot stamping process which is implemented by DED. The combination of the described methods allows for manufacturing hot stamping tools with near-surface cooling channels and a global or local adjustment of the surface properties of the tools.

Air jet impingement cooling for hot stamping dies

The strength of panel sheets can be increased by 2–3 times after hot stamping, and for high quality of hot stamped sheets, uniform cooling rate on the die face to quench the panel is needed. However, it is difficult for cooling water channels, commonly used for hot stamping dies, to dissipate heat evenly. In addition, dies are easily corroded by contact with water at high temperature. Thus, this study considers air cooling for the quenching process. The airflow can be applied to the sheet directly during the forming stage to cool and quench, eliminating difficulties with cooling channel design and providing more uniform cooling. Simulations and experiments were used to test the feasibility of air cooling for hot stamping with CSC-15B22 steel. A simple hot stamping tool with flat die face and either water or air cooling channels was used for testing, and results show that the both direct and indirect airflow can be used for hot stamping. With indirect air cooling, the cooling rate is sufficient for complete martensite transition, though the cooling rate is 40% lower than with indirect water cooling. To improve cooling efficiency of the air cooling system and obtain more uniform cooling, an air jet impingement cooling system (direct airflow cooling) was also tested. Results show that a similar cooling rate can be obtained in both indirect water cooling and the air jet impingement cooling. Thus, it is possible for air jet impingement cooling to provide hot stamping superior to water cooling. These results can be a reference for the design of new cooling systems for hot stamping dies.

Influence of Conformal Cooling Channel Parameters on Hot Stamping Tool and Press-Hardening Process

Press hardening is a technology increasingly used in the automotive industries to produce high-strength structural parts in boron steel. The part strengthening is due to a martensitic microstructure, which is obtained during the quenching phase of the process by imposing cooling rates typically higher than 27 K/s. To improve cooling efficiency, conformal cooling channels (CCC) are increasingly adopting thanks to opportunities offered by additive manufacturing (AM) technologies in combination with the development of powders with high thermal conductivity.In this work a methodology for the design of CCC inside hot stamping tools is presented and the press hardening of an automotive B-Pillar in 22MnB5 has been used as case study. The proposed methodology provides for an optimization of the distance between the cooling channels (p) and distance between the tool surface and the cooling channel center (d).

Tribological and Thermal Behavior of Laser Implanted Tool Surfaces for Hot Stamping AlSi Coated 22MnB5 Sheets

In the automotive industry, the development of electrically powered vehicles has become a major forward-looking topic. For improving the range and thus the efficiency of electric cars, lightweight construction has gained even more importance. In this regard, hot stamping has been established as a suitable and resource efficient process to manufacture high-strength and lightweight body-in-white components. This method combines hot forming and quenching of boron-manganese steel 22MnB5 in a single process step. As a result, complex structures with thin sheet thicknesses and high ultimate tensile strength up to 1500 MPa are generated. However, the use of lubricants is not possible at elevated temperatures, which subsequently leads to high thermo-mechanical tool stresses. As a side effect, high friction and severe wear occur during the forming process, which affect the resulting part quality and maximum tool life. Therefore, the aim of this study is to improve the tribological performance of hot stamping tools by using a laser implantation process. This technique is based on manufacturing highly wear resistant, separated and elevated structures in micrometer range by embedding hard ceramic particles into the tool material via pulsed laser radiation. As a result, highly stressed areas on the tool surface can be modified locally, which in turn influence the tribological and thermal behavior during the forming process. In this regard, laser implanted and conventionally tool surfaces were investigated under hot stamping conditions. A modified pin-on-disk test was used to analyze the friction coefficient and occuring wear mechanisms. Furthermore, quenching tests as well as hardness measurements were carried out to gain in-depth knowledge about the cooling behavior of the modified tool surfaces and its impact to the resulting mechanical part properties.

Selection of laser processing parameters for hot stamping tools based on mathematical planning of the experiment

The results of experimental studies to determine the influence of the technological parameters of laser processing of 3Kh3M3F die steel on the service life of the working tool of a hot-stamping machine designed for hot stamping of rolling bearing rings are presented. Development of a mathematical model of the tool's resistance to laser radiation parameters was carried out by the method of mathematical planning of the experiment using a full factorial experiment. Durability of the die tool N (in thousand forgings) was chosen as an optimization criterion, while dimensionless (generalized) parameters of the speed V* and power P* of laser processing, as well as the overlap coefficient of laser tracks S* were proposed to use as factors. The obtained mathematical model of the resistance of laser-hardened die tools for hot deformation is used for the development and optimization of laser quenching modes without melting the surface of the ejectors in order to increase their thermal fatigue strength. Due to the fact that dimensionless factors containing the thermophysical characteristics of the die tool material are used in the planning of the experiment, the developed mathematical model can be used to assess the operational resistance of other heat-resistant tool steels of the martensite class.

Study on new hot stamping tool with low cost and high cooling efficiency

A new manufacturing method of hot stamping tool with CCC is proposed in this study. A pair of optimal hot stamping tool should have long service life, low manufacturing cost and good cooling performance simultaneously. The CuCrZr alloy and Fe-based alloy is selected as interlayer layer (IL) and surface layer (SL) respectively. The maximum stress of new tool was less than the yield strength of the corresponding welding materials, which demonstrated that new tool manufactured by surfacing technology could satisfy the strength requirement and deserve further research.

Analysis of Hot Stamping Tool Cooling System-A Case Study.

One of the important steps in the design of hot stamping tools is the analysis of their cooling system. This article presents an authorial, two-stage approach to solving this problem. The first stage consisted of a series of simulations of the hot stamping process in the Autoform package, with initial selection of shape and arrangement of cooling channels. These results allowed for the design of the tool for which the coupled thermal-flow analysis was carried out. The correctness of the adopted design assumptions has been confirmed by experimental tests. A trial series of drawpieces made in production conditions meet the requirements for hardness, mechanical properties, and appropriate microstructure. The presented procedure has become the practice of the drawpiece producers.

A case study on the design of a hot stamping tool with conformal cooling channels

Additive manufacturing (AM) can be used to manufacture tool inserts with cooling channels conforming to the shape of the part to be produced. This results in shorter cycle times and improved quality. In the hot stamping industry, there is need to reduce cycle time and improve quality. The process differs from injection moulding since it involves larger tool inserts, high temperature and forces. In hot stamping tools, the cooling channels are in the form of straight drilled channels. These channels do not have the capacity to cool complex shaped parts consistently due to restrictions in machining. There is need for further studies on the design of hot stamping tools with AM-based cooling channels. The aim of this paper is to propose a method for designing hot stamping tools with conformal cooling channels. The method involves firstly evaluating the part to decide whether it is suitable for AM application, determining conformal cooling parameters and developing alternative layouts. This is followed by selection of the optimum layout and obtaining the cooling system parameters.

Study on Microstructure and Numerical Simulation of Tailored Hot Stamping Tools Refabricated by Surfacing Co-Based Alloy

Heated tailored hot stamping tool would suffer serious wear at high temperature after a period of use. An economical and efficient repair method is proposed to refabricate the failed tool. In this study, the physical properties and microstructure of the refabricated tool were revealed. 5CrNiMo substrate and Co-based surfacing layer could form excellent metallurgical bonding. Co-based surfacing layer exhibited higher hardness and high-temperature strength. Temperature field and stress field of 5CrNiMo homogenous tool and 5CrNiMo+Co-based tool for heated tailored tool were simulated, the results showed that the highest temperature and maximum stress always occurred at the time of t = 3s or t = 4s. The refabricated tool could better satisfy the high-temperature strength requirement than homogenous 5CrNiMo. At last, the feasibility of surfacing Co-based alloy on 5CrNiMo tool was verified, and B-pillar also was produced successfully by the refabricated tool.

Investigation of the thermal and tribological performance of localized laser dispersed tool surfaces under hot stamping conditions

In the automotive industry, hot stamping has been established as a key technology for manufacturing safety-relevant car body components with high strength-to-weight ratio. However, hot stamping tools are stressed by cyclic thermo-mechanical loads, which leads to severe wear and high friction during the forming operation. Consequently, the quality of the parts, the durability of the tools and the efficiency of the process are negatively affected. Within the scope of this work, a promising approach named laser implantation process has been investigated for improving the tribological behavior of hot stamping tools. This technique enables the fabrication of highly wear resistant, separated and elevated micro-features by embedding hard ceramic particles into the tool via pulsed laser radiation. Hence, highly stressed tool areas can be modified, which influences the thermal and tribological interactions at the blank-die interface. To clarify these cause-effect relations, numerical simulations, quenching tests as well as tribological investigations have been conducted. In this context, laser-implanted tools reveal a significantly improved tribological performance while offering the possibility to adjust the thermal properties within hot stamping. Based on these results, a tailored tool modification can be pursued in future research work, in order to enhance the effectiveness of hot stamping tooling systems.

CAE Modeling of Hot Stamping Tool Wear

To assess the wear of the hot stamping tool, a model of punch deformation under the influence of thermal and mechanical loads was developed. The shape and relative position of the punch and die were designed to localize maximum contact loads at a predetermined location. It was assumed that the punch wear is caused by a removal of the surface metal layer, due to decreasing its mechanical strength during hot plastic deformation. Simulation was performed by the finite element method in the Deform software package. A temperature field and wear pattern in the contact zone were calculated. With respect to the loads typical for hot forging railway wheels, the comparison showed that the wear in a numerical experiment is of the same level as the actual value of wear.

Laser Implantation of Niobium and Titanium-Based Particles on Hot Working Tool Surfaces for Improving the Tribological Performance within Hot Stamping

Within the scope of this work, a new approach named laser implantation process has been investigated, in order to improve the tribological performance of hot stamping tools. This surface engineering technology enables the generation of dome-shaped, elevated and highly wear resistant microfeatures on tool surfaces in consequence of a localized dispersing of hard ceramic particles via pulsed laser radiation. As a result, the topography and material properties of the tool and thus the tribological interactions at the blank-die interface are locally influenced. However, a suitable selection of hard ceramic particles is imperative for generating defect-free surface features with a high share of homogenously disturbed particles. For this purpose, different niobium (NbB2 and NbC) as well as titanium-based (TiB2 and TiC) materials were embedded on hot working tool specimens and subsequently analyzed with regard to their resulting shape and mechanical properties. Afterwards, modified pin-on-disk tests were carried out by using conventional and laser-implanted tool surfaces, in order to evaluate the wear and friction behavior of both tooling systems.

Thermomechanical analysis of additively manufactured bimetallic tools for hot stamping

Abstract A comparison between a conventional AISI H13 hot stamping tool and a bimetallic tool consisting of an AISI 1045 core and a laser-deposited AISI H13 coating is performed. In order to analyze the performance of bimetallic tools, the material compatibility and quality of the coating are analyzed. Besides, the mechanical properties are evaluated and compared with those of the conventional tool, obtaining mechanically equivalent results. Nevertheless, the real conductivity of the laser deposited AISI H13 is found to be 16 % lower than the theoretical value. Hence, a thermal model of the hot stamping process is developed, and the performance of various coating thicknesses is evaluated. Results show that, in the present case study, an AISI 1045 tool with a 1 mm AISI H13 coating ensures the mechanical properties and reduces the cycle time by 44.5 % when compared to a conventional AISI H13 tool.

Localized Laser Dispersing of Titanium-Based Particles for Improving the Tribological Performance of Hot Stamping Tools

Within the scope of this work, a new surface engineering technology named laser implantation has been investigated, in order to improve the tribological performance of hot stamping tools. This technique is based on manufacturing highly wear-resistant, separated, and elevated microfeatures by embedding hard ceramic particles into the tool surface via pulsed laser radiation. Hence, the topography and material properties of the tool are modified, which influences the thermal and tribological interactions at the blank-die interface. To verify these assumptions and to clarify the cause–effect relations, different titanium-based particles (TiB2, TiC, TiN) were laser-implanted and subsequently analyzed regarding to their geometrical shape and mechanical properties. Afterwards, quenching tests as well as tribological experiments were carried out by using titanium-diboride as the most promising implantation material for reducing the tribological load due to high hardness value of the generated implants. Compared to conventional tooling systems, the modified tool surfaces revealed a significantly higher wear resistance as well as reduced friction forces while offering the possibility to adjust the thermal interactions at the blank-die interface. Based on these results, a tailored tool surface modification can be pursued in future research work, in order to enhance the effectiveness of the hot stamping technology.

Fundamental research and numerical simulation of new hot stamping tool manufactured by surfacing technology

A novel manufacturing method of hot stamping tool is proposed in this study. The four preselected materials were welded on the specimens by using tungsten inert gas arc welding. High-temperature compressive yield strength, heat conductivity, thermal expansion coefficient, and microstructures of welding materials were experimentally investigated. CuCrZr and Fe-based alloy were selected as the intermediate layer (IL) and surface layer (SL) of new hot stamping tool respectively. Moreover, the simulation results indicated that the dwell time of new tool (NT) can be saved by 25% in comparison to traditional tool (TT); the temperature of traditional and new tool would reach dynamic thermal equilibrium after 4 and 5 stamping circles respectively; the temperature and stress of new tool were always lower than traditional tool; the maximum stress of new tool in each area was always less than the yield strength of the corresponding welding materials, which demonstrated that new tool manufactured by surfacing technology could satisfy the strength requirement and deserve further research in the future.

A comparison study on wear characteristics of Ni-based, Co-based and Fe-based alloys for heated hot stamping tools manufactured by surfacing technology

To meet the demanding requirement of ultra-high-strength steel parts with tailored properties, heated hot stamping tools are becoming widely used. However, due to severe working conditions, wear on tool surface is serious. Therefore, the service life of heated hot stamping tools is limited. In this paper, with the purpose of improving the service life of heated hot stamping tools, we aim to find a suitable alloy with better wear-resistant properties to manufacture heated hot stamping tools by surfacing technology. Thus, wear comparison tests between SDCM steel and specimens made of Ni-based, Co-based and Fe-based alloys were conducted under 500 °C and 180 N normal load to determine which kind of alloy has the best wear-resistant properties. Specifically, the mechanical properties, worn volume, worn morphologies and wear mechanisms of test materials were investigated. The results indicate that the Fe-based alloy has the highest compressive yield strength and hardness, but the Co-based alloy has the lowest worn volume, which is 0.49 mm3 at 20 min and 0.70 mm3 at 90 min. And there is no obvious positive relationship between the mechanical properties and the wear-resistant properties of the test materials under the test conditions. The common wear characteristic of the four test materials is that their main wear mechanisms are oxidation wear. The Co-based alloy has the moderate mechanical properties, and its wear mechanisms are oxidation wear and adhesive wear; thus, shear tearing, material transfer and severe abrasive wear are avoided, and the oxide layers adhere to its worn surface instead of spalling. Therefore, the Co-based alloy manifests the best wear-resistant properties and is chosen as the surfacing materials for heated hot stamping tools.